Communication systems are known to support wireless and wire lined communications between wireless and/or wire lined communication devices. Such communication systems range from national and/or international cellular telephone systems to the Internet to point-to-point in-home wireless networks. Each type of communication system is constructed, and hence operates, in accordance with one or more communication standards. For instance, wireless communication systems may operate in accordance with one or more standards including, but not limited to, IEEE 802.11, Bluetooth, advanced mobile phone services (AMPS), digital AMPS, global system for mobile communications (GSM), code division multiple access (CDMA), wireless application protocols (WAP), local multi-point distribution systems (LMDS), multi-channel-multi-point distribution systems (MMDS), and/or variations thereof.
Depending on the type of wireless communication system, a wireless communication device, such as a cellular telephone, two-way radio, personal digital assistant (PDA), personal computer (PC), laptop computer, home entertainment equipment, et cetera communicates directly or indirectly with other wireless communication devices. For direct communications (also known as point-to-point communications), the participating wireless communication devices tune their receivers and transmitters to the same channel (e.g., one of the plurality of RF carriers of the wireless communication system) and share information over that channel. For indirect wireless communications, each wireless communication device communicates directly with an associated base station (e.g., for cellular services) and/or an associated access point (e.g., for an in-home or in-building wireless network) via an assigned channel. To complete a communication connection between the wireless communication devices, the associated base stations and/or associated access points communicate with each other directly, via a system controller, via the public switch telephone network, via the internet, and/or via some other wide area network.
Each wireless communication device includes a built-in radio transceiver (i.e., receiver and transmitter) or is coupled to an associated radio transceiver (e.g., a station for in-home and/or in-building wireless communication networks, RF modem, etc.) to participate in wireless communications. As is known, the receiver receives RF signals, removes the RF carrier frequency from the RF signals via one or more intermediate frequency stages, and demodulates the signals in accordance with a particular wireless communication standard to recapture the transmitted data. The transmitter converts data into RF signals by modulating the data in accordance with the particular wireless communication standard and adds an RF carrier to the modulated data in one or more intermediate frequency stages to produce the RF signals.
As is also known, the receiver is coupled to the antenna and includes a low noise amplifier, one or more intermediate frequency stages, a filtering stage, and a data recovery stage in many designs. The low noise amplifier receives an inbound RF signal via the antenna and amplifies it. The one or more intermediate frequency stages mix the amplified RF signal with one or more local oscillations to convert the amplified RF signal into a baseband signal or an intermediate frequency (IF) signal. As used herein, the term “low IF” refers to both baseband and intermediate frequency signals. A filtering stage filters the low IF signals to attenuate unwanted out of band signals to produce a filtered signal. The data recovery stage recovers raw data from the filtered signal in accordance with the particular wireless communication standard.
To carry out filtering at the intermediate frequencies, surface acoustic wave filters (SAW) are commonly used. The SAW filters have the drawback, however, of being bulky, heavy and expensive. Additionally, the SAW filters require low impedance matching thereby resulting in high power consumption. Because they are often powered by battery, portable wireless communication devices are not readily adaptable for such systems in that they are required to be inexpensive, light and consume lower amounts of power. Thus, there is a need to design transceiver systems that eliminate the use of intermediate frequency filters.
An alternate approach to using a higher intermediate frequency that requires the SAW filters is to convert the RF signal to an intermediate frequency sufficiently low to allow the integration of on-chip channel selection filters. For example, some narrow band or low data rate systems, such as Bluetooth, use this low intermediate frequency design approach.
One problem of using low intermediate frequencies, however, is to satisfy the image rejection requirements for the systems. The image rejection requirement for the down-conversion is hard to meet and is usually limited to about −40 dB. Thus, this low intermediate frequency approach is limited for narrow band or low data rate systems. Wide band or high data rate systems require an intermediate frequency that is not low enough for the integration of channel selection filters given the technology that is available today for semiconductor processes. There is a need, therefore, for a wireless transceiver system that allows for full integration on-chip of circuit designs that support high data rate and wideband communications. Stated differently, there is a need for wireless transceiver systems formed on an integrated circuit that have the capability to convert between baseband and a specified RF band in a single step to avoid the image rejection problem discussed above. Thus, it is desirable to design direct conversion radio transceivers to allow a transceiver to be built on one integrated circuit.
As the demand for enhanced performance (e.g., reduced interference and/or noise, improved quality of service, compliance with multiple standards, increased broadband applications, et cetera), smaller sizes, lower power consumption, and reduced costs increases, wireless communication device engineers are faced with a very difficult design challenge to develop such a wireless communication device. Typically, an engineer is forced to compromise one or more of these demands to adequately meet the others. For example, if a device is in power saving mode wherein at least some circuit elements are powered down while the receiver is not receiving data, the powered down circuit element must be powered back up and must reach a steady state within a specified period.
While these design trends exist, there is also constant development being pursued with the goal of increasing data rates. As data rates are increased, circuits have less time to reach a steady state for routine operations. This problem is compounded in those systems in which a circuit must be activated upon receipt of a data frame or packet. As data throughput rates increase, a circuit or radio receiver must reach a steady state during a short preamble period before data packet arrives. As the data rates are increased, however, the preamble period is decreased thereby decreasing the time allowed for a circuit or device to settle or reach a steady state. Thus, utilizing conventional designs, many circuit elements cannot be powered down because their settle time is too large to reach steady state during the shortened preamble periods.
Accordingly, there is a need for radio receivers and, more particularly, radio receiver circuits, to reach a steady state for increasingly shorter preambles or whenever a circuit is merely transitioned from an inactive state to an active state. Moreover, direct conversion transceivers are known to create a DC offset that requires removal prior to amplification by the receiver circuitry. Given the timing restrictions, however, it is difficult for a circuit and, more particularly, a low pass filter, that, in conjunction with a variable gain amplifier including a high pass filter is used to remove the DC offset to settle in the required amount of time under circuit designs. Therefore, a need exists for a circuit that can readily and quickly remove the DC offset and that can also settle within a required settle time for a high throughput communication system.